The present invention relates to a spark plug carrying out a spark discharge between a center electrode and a grounding electrode and also relates to a method of manufacturing such spark plug.
A spark plug is generally utilized under a sever thermal condition (high thermal load), and hence, in order to suppress spark consumption due to progressing of internal oxidation of the grounding (ground or earthing) electrode, a nickel (Ni-)base alloy such as Inconel (Trade Mark Name) having an excellent heat resisting and acid-proof property has been utilized for a material of the grounding electrode. On the other hand, a main metal fitting (metal element) holding, in an insulated manner, the central electrode is generally formed of a low carbon steel.
The main metal fitting of the low carbon steel is effected with plating for the purpose of corrosion protection such as disclosed, for example, in Japanese Patent Laid-open Publication No. 1996-236263, which provides a spark plug with improved heat resistance by carrying out plating after the grounding electrode has been welded to the main metal fitting. The grounding electrode is preliminarily bent to prevent cracking before the plating treatment from causing. Furthermore, Japanese Patent Laid-open Publication No. 2001-68250 also provides a spark plug and a manufacturing method thereof and discloses that a main metal fitting and a grounding electrode are integrated, and in this state, zinc plating is effected thereto except a tip end portion of the grounding electrode or the grounding electrode side is masked and the main metal fitting is plated after the welding of the main metal fitting to the grounding electrode. Thereafter, the zinc plating (plated) layer formed to the tip end side of the grounding electrode is peeled and removed, and the thus exposed surface is then welded with a high melting point metal to thereby form an ignition portion.
However, in the case that the main metal fitting and the grounding electrode are plated after the welding, when the grounding electrode is bent for the adjustment of a gap between the center electrode and the grounding electrode, there may cause a case that the plated layer or film on the grounding electrode may be peeled off and the center electrode and the grounding electrode are short-circuited through the peeled plating film or layer, which may cause a flame-off (misfire) phenomenon.
On the other hand, in the case that the masking is effected to the grounding electrode after the welding process, since the grounding electrode is not completely covered by the masking, there causes a fear that a plated film or layer having a height of about 1-2 mm adheres on the welded side end of the grounding electrode (such as shown in FIG. 2). In such case, the adhering plated layer is peeled by a thermal stress during engine operation and flying sparks or lifted flame may occur, causing a fear of an ignition failure.
Moreover, in advancing technology in which a life time of the spark plug is elongated, there is a fear of causing peel-off of the plated layer in a long time use even if little amount of the plated layer remains.
An object of the present invention is to substantially eliminate defects or inconveniences encountered in the prior art mentioned above and to provide a spark plug having an improved structure capable of surely preventing flame-off or ignition failure caused by peeling off the plated layer or film on a grounding electrode.
This and other objects can be achieved by providing a spark plug comprising:
a main metal fitting;
a center electrode mounted to and held by the main metal fitting in a manner insulated therefrom; and
a grounding electrode having one end welded to the main metal fitting and another end opposing to the center electrode,
wherein a welding burr is formed to the welded portion of the main metal fitting and the grounding electrode so as to have an area expanding outside the grounding electrode, and only the main metal fitting is plated, and the grounding electrode and the welding burr are not plated.
According to this aspect, since the main metal electrode is plated, the main metal fitting can be prevented from corroding and, on the other hand, since the ground electrode and the welding burr are not plated, misfire or ignition failure caused by the peeling of a plated layer or film formed to the grounding electrode, as in the conventional structure, can be effectively prevented from causing.
In a preferred embodiment of the above aspect, the main metal fitting is plated at a portion except for the welded portion, and the welded portion includes a non-plating (non-plated) portion which is entirely covered by the grounding electrode and the welding burr. The non-plating portion of the main metal fitting is formed by once effecting the plating treatment to the entire surface of the main metal fitting and then peeling, by polishing, the plated layer of a portion which is welded thereafter to the grounding electrode. The non-plating portion of the main metal fitting may be formed by effecting a masking treatment to the main metal fitting. The grounding electrode may be preferably formed from an Ni-base alloy.
According to such preferred embodiment, since only the portion to be welded of the main metal fitting is plated, the welding strength between the main metal fitting and the grounding electrode can be ensured in addition to the prevention of the corrosion to the main metal fitting. The welding burr covers the non-plated (non-plating) portion of the main metal fitting, so that the corrosion of the non-plated portion can be prevented. The use of the Ni-base alloy for the grounding electrode makes free from plating.
In a further preferred embodiment, the welded portion includes a non-plating portion and S/A value is in a range of 0.7xe2x89xa6S/Axe2x89xa61.5 in which symbol S is an area of the non-plating portion of the main metal fitting and symbol A is an area of the welded side end surface of the grounding electrode.
In the case where the area S of the non-plating portion is too small, it is difficult to ensure a necessary welding strength, and the grounding electrode is hence easily peeled off from the welding interface at the time of adjusting the spark gap. In the examination of the inventor, it was found that the necessary strength could be ensured by setting the S/A value to 0.7xe2x89xa6S/A.
On the other hand, in the case where the area S of the non-plating portion is considerably larger than the welding side area A of the grounding electrode, in order to cover the non-plating portion with the grounding electrode and the welding burr, it is necessary to make large the welding burr by excessively setting the welding condition. However, it was found that when the welding burr becomes excessively large, a horizontal flying spark was easily generated to the welding burr. It was also found that such defect could be effectively eliminated by setting the S/A value to S/Axe2x89xa61.5.
In another aspect of the present invention, there is also provided a method of manufacturing a spark plug including a main metal fitting, a center electrode supported by the main metal fitting in a manner insulated therefrom and a grounding electrode having one end to which the main metal fitting is welded and another end opposing to the center electrode,
wherein, only the main metal fitting is plated to a portion except the portion to be welded to the grounding electrode and said grounding electrode is then welded to the main metal fitting.
In a preferred embodiment of this aspect, the welding is effected under oxygen free atmosphere.
According to this aspect, oxidation coloring which may be caused at the welding time can be prevented.
Furthermore, substantially the same advantageous functions and effects as those mentioned above with reference to the preferred embodiment of the spark plug of the present invention can be achieved also by this manufacturing method.
The nature and further characteristic features of the present invention will be made further clear from the following descriptions made with reference to the accompanying drawings.